There is no admission that the background art disclosed in this section legally constitutes prior art.
Solid phase microextraction (SPME) is a useful solvent-free sampling technique. SPME has gained widespread acceptance and use in laboratories due to the fact that it is a solvent-less extraction technique, its mode of operation is relatively simple and easy to automate, and sampling and sample preparation are combined into one single step.
Generally, SPME uses a fiber that is coated with a stationary phase material, such as a liquid polymer, solid sorbent, or a mixture of both. Equilibrium is established between an analyte and the coating material when the fiber is exposed to a solution, which allows the technique to be applied to both headspace and direct-immersion sampling. When SPME is coupled with gas chromatography (GC), the analytes are desorbed from the fiber coating by thermal desorption in the injection port of the GC.
The development of new coating materials for SPME has flourished in the past decade as the technique continues to gain wide-spread popularity. The need for new coating materials is underscored by the fact that SPME methods must achieve high sensitivity and selectivity. The coating material must be designed to be resistant to extreme chemical conditions, such as pH, salts, organic solvents, and modifiers.
To achieve long fiber lifetimes, the coating should be thermally stable to avoid excessive losses during the high temperature desorption step, while also maintaining physical integrity of the film.
As SPME methods become more developed in sampling complicated environmental and biological matrices, structural tunability is a desirable means of modulating specific properties of the coating material while retaining others.
Solid phase microextraction (SPME) and stir bar sorptive extraction (SBSE) are two solvent-free sampling techniques in which sampling and sample preparation are combined into one single step.
SPME generally uses a fused silica fiber that is coated with an absorbent or adsorbent coating material, typically polydimethylsiloxane (PDMS), polyacrylate, or carbowax divinylbenzene. Depending on the mode of extraction (headspace or direct immersion), the analytes are sampled due to their partitioning to the coating material, typically under equilibrium conditions. The analytes are desorbed from the fiber using either thermal desorption (i.e., injection port of a gas chromatograph) or by solvent desorption (i.e., solvent chamber coupled to a high performance liquid chromatography).
SBSE operates in a similar manner to SPME but differs in the type of support and the amount of coating material employed in the extraction. In SBSE, the analytes are extracted into a thick polymer coating on a magnetic stir bar. The amount of coating material in SBSE is ˜50-250 times larger than SPME, which produces a distinct sensitivity enhancement.
Polymer coating materials used in SBSE have largely focused on PDMS, although there has been a report of incorporating sol-gel technology into the PDMS coating material. The development of new coating materials for SPME has flourished in the past five years as the technique has gained wide-spread popularity.
Ionic Liquids (ILs)
Ionic liquids (ILs) are a class of compounds that can be tailor synthesized to exhibit unique solvent properties while retaining many green characteristics. Ionic liquids (IL) and their polymerized analogs constitute a class of non-molecular, ionic solvents with low melting points. Also known as liquid organic, molten, or fused salts, most ILs possess melting points lower than 100° C. Many ILs are comprised of bulky, asymmetric N-containing organic cations (e.g., imidazole, pyrrolidine, pyridine) in combination with any wide variety of anions, ranging from simple inorganic ions (e.g., halides) to more complex organic species (e.g., triflate).
ILs have negligible vapor pressures at room temperature, possess a wide range of viscosities, can be custom-synthesized to be miscible or immiscible with water and organic solvents, often have high thermal stability, and are capable of undergoing multiple solvation interactions with many types of molecules.
These interactions and properties of ILs now make molten organic salts and imidazolium and pyrrolidinium-based ILs a useful class of stationary phase materials in gas chromatography (GC). In particular, the separation selectivity and thermal stability can be altered by changes to the cation and/or anion, polymerization and immobilization of the IL, and by blending different ILs to form stationary phases with varied composition.
However, many classes of neat ILs have a strong propensity to flow off the fiber when employing moderate to high desorption temperatures (200° C. and above) and desorption times of 4 minutes or longer. Also, several complications arise from the loss of the IL during the desorption step: (1) a compromise between the desorption time and temperature must be achieved; (2) any IL present in the injection port will contaminate the liner, and the line must be constantly removed and cleaned to prevent unwanted IL-decomposition products to appear as chromatographic ghost peaks; and, (3) the support needs to be re-coated with the IL, thereby making it inconvenient while also decreasing fiber-to-fiber reproducibility.